Process for cooling metal



United States Patent 3,323,577 PROCESS FOR COOLING METAL Joseph A. Clumpner, Bethany, Conn., assignor to Olin Mathicson Chemical Corporation, a corporation of Virginia No Drawing. Filed May 5, 1965, Ser. No. 453,505 13 Claims. (Cl. 1651) The present application is a continuation-in-part of United States patent application Ser. No. 156,119, filed Nov. 30, 1961 and now abandoned.

The present invention relates to a method for the rapid removal of heat from high temperature copper and aluminum articles. More particularly, the present invention relates to a method for rapidly cooling copper and aluminum articles which have been heated in connection with metal processing and fabricating procedures.

There are numerous processes in various industries where the heating or cooling of metals is important. In the metal industry heating, and the transfer of heat, plays a vital role in the fabricating steps of casting, preheating, hot rolling, annealing, solution heat treating, quenching, and aging.

For example, copper and aluminum strip, sheet or eX trusions frequently require the rapid cooling from elevated temperatures in excess of 700 F. to ambient temperatures. These processing steps are normally conducted on an assembly line basis so that the more rapid the cooling the more economical the operation.

In addition, these procedures are frequently carried out in a moving line so that it is highlydesirable to develop a process which is capable of extremely fast removal of heat in order to limit the size and cost of quenching facilities.

In procedures such as these, the sheet, strip, or extrusion must be cooled to ambient temperatures in a matter of seconds. In these procedures, a period of time of a few seconds is extremely significant and any procedure which can attain cooling in a period of time a few seconds shorter than existing procedures represents a considerable or commercial saving.

Illustrative of the problems frequently encountered in the art is in the cooling of rapidly moving strip. For example, consider copper or aluminum strip one inch thick moving on a production line at a rate of ten feet per second, with the strip initially at 1000 F. to be cooled to less than 300 F. Using conventional quenching methods, such as spraying the strip with water using spray nozzles operating at a pressure of 60 psi. or moving the strip submerged through a tank of agitated cold water, at least eight seconds are required to accomplish the desired cooling. Since the strip speed is feet per second, in eight seconds the strip travels a distance of 80 feet. This means that a conventional quenching line for the above hot strip must be at least 80 feet long. Obviously from a practical point of view it is highly desirable to shorten the length of the cooling line and obviously an increase in the cooling rate will shorten the cooling line and result in a considerable financial saving.

Accordingly, it is a principal object of the present invention to provide a method for increasing the rate of heat removal from the surface of a copper or aluminum article.

It is a further object of the present invention to provide a method as aforesaid for rapidly cooling the entire surface of a copper or aluminum article, particularly in sheet, strip or extrusion form.

It is a further object of the present invention to provide a method as aforesaid which is capable of extremely rapid cooling of the entire surface of copper or aluminum articles from temperatures in excess of 700 F. to ambient temperatures, in particular to devise a process which effects said cooling in a matter of seconds.

It is a further object of the present invention to devise such a process which is readily adaptable to use on a commercial scale.

Further objects and advantages of the present invention will appear hereinafter.

In accordance with the present invention, it has now been found that the foregoing objects and advantages may be readily accomplished. Accordingly, there has now been devised a process for cooling copper and aluminum articles which comprises:

A. Providing said metal wherein the entire surface thereof is at a temperature in excess of 700 F., and preferably in excess of 800 F.;

B. Spraying substantially the entire surface thereof with an aqueous jet of high velocity liquid in the form of droplets to penetrate the vapor barrier formed at the surface of said metal by vaporization of said liquid, wherein (1) said jet is at a distance of from 6 to 24 inches from the surface of the metal,

(2) said jet is sprayed at a pressure of 150 to 600 pounds per square inch and preferably 200 to 400 pounds per square inch,

(3) said droplets have a velocity of from to 300 feet 'per second, and

(4) the coolant flow rate is at least 20 gallons per minute per square foot of surface over the area of impact;

C. Thereby cooling the entire surface of said metal at a rate of at least F. per second. The present invention is preferably applicable to copper or aluminum sheet, strip or extrusions.

Unfortunately, one of the factors which limits the rate of transfer of heat through a solid surface to a liquid cooling media is the buildup, in contact with the surface, of a vapor film which serves as a barrier to the transfer of heat. This vapor film is formed by partial vaporization of the liquid cooling medium at the high temperature surface, and a low rate of heat transfer through this film drastically reduces the rate of cooling which can be achieved using liquid media.

Attempts to overcome the low heat transfer rates by spraying water on hot surfaces above 700 F. through conventional spray cooling nozzles at pressures up to 70 pounds per square inch have not produced significantly higher cooling rates over those achieved by immersion quenching, that is, by direct immersion of the heated object in quiescent or moving bulk liquid.

In accordance with the present invention, heat is removed from the entire surface of a copper or aluminum article having an initial high temperature above the vaporization temperature of a cooling liquid by subjecting the surface of the material to the action of small radius jets or droplets of the cooling liquid at a very high velocity sufficient to penetrate the vapor film formed at said surface by vaporization of said liquid. Quite surprisingly it has now been discovered that it is possible to cool the entire surface of a large copper or aluminum article from temperatures well above the boiling point of the cooling liquid at a rate significantly greater than the rate of cooling achieved by immersion quenching. This rate of cooling greater than the immersion quenching rate can be attained because the particles or jets of liquid are given a linear velocity which is sufiiciently high to penetrate and pass through the vapor film formed at the surface of the body.

It has now been found that super quenching, i.e., a rate of cooling in excess of the rate attained by direct immersion, can be produced only where the liquid is delivered to the surface to be cooled by spaced liquid jets or droplets of small radius, and where the linear velocity of the liquid delivered to the high temperature surface is sufficiently high to permit the cooling liquid to penetrate and pass through the vapor film continuously formed at the surface of the body. The high linear velocity of the jet delivering liquid to the surface is a key to the production of very high cooling rates in excess of the cooling rate obtainable by immersion quenching in either still or flowing bulk cooling liquid. For example, at a surface temperature of 900 F., a surface cooling rate of about 300 F. per second is achieved in accordance with the present invention when the aqueous coolant is sprayed at a pressure of 200 psi. In sharp contrast, however, cooling rates achieved by immersion quenching or by spray quenching at pressures up to 70 p.s.i., are limited to about 50 F. per second when the surface temperature of the object being quenched is 900 F.

In accordance with the present invention it has been found that the process conditions are critical in achieving the super quenching elfect which is the surprising feature of the present invention.

The prevent invention is applicable to the cooling of aluminum or copper articles wherein the entire surface thereof is at a temperature in excess of 700 F, and preferably in excess of 800 F. At temperatures below 700 F., the vapor barrier formed by aqueous coolants becomes less stable and in general high cooling rates may be achieved at pressures below 150 p.s.i. At even lower surface temperatures below 250 F., high cooling rates may be achieved by immersion quenching and there is no particular advantage in quenching with high velocity droplets. On the other hand, temperatures in excess of 700 F. are especially troublesome in view of the vapor barrier problem. It is in this temperature range that the surprising advantages of the present invention are found.

The present invention is applicable to copper and aluminum articles and alloys of these metals. These materials present particular cooling problems in view of the fact that they are high thermal conductivity materials and have high thermal diffusivities. Thermal diffusivity is a material property defined as thermal conductivity divided by heat capacity per unit volume. The thermal diffusivity of a material gives an accurate indication of how rapidly heat diffuses from the interior of a material to the surface when an object is quenched.

For example, with copper and aluminum the thermal diffusivity values are about 0.14 square inch per second. As a comparison, the thermal diffusivity for mild steel is about 0.02 square inch per second. These values show that heat flows about seven times faster within aluminum and copper articles than it does in mild steel objects of the same shape.

Because of this more rapid internal heat fiow, it is more diificult to rapidly cool the surfaces of copper and aluminum articles than to cool the surface of mild steel. As heat is removed from the surface of the copper and aluminum articles, heat from the interior rapidly travels to the surface and maintains that surface at approximately the same temperature as the interior of the object. In contrast, as heat is removed from the surface of a mild steel object, for example, heat flows from the interior at a slower rate, thus making it easier to cool the surface to a temperature substantially lower than the interior temperature.

Because of the fact that the high thermal diffusivity of copper and aluminum operates to maintain the surface temperature at a high level, rapid lowering of mean temperature requires a quenching process that achieves very high and in fact surprising heat transfer rates at surface temperatures greater than 700 F. The process of the present invention eminently satisfies this requirement and achieves a surprisingly high cooling rate.

In accordance with the present invention, substantially the entire surface of the article is sprayed with an aqueous jet of high velocity liquid in the form of small droplets to penetrate the vapor barrier. The jet is spaced at a distance of from 6 to 24 inches from the surface of the metal and the jet is sprayed at a pressure of from 150 to 600 pounds per square inch.

The preferred distance from the metal is 6 to 15 inches. At greater distances the velocity of the droplets tends to slow down and the droplets become less effective in penetrating the vapor barrier. The preferred operating pressure is from 200 to 400 pounds per square inch. Achieving pressures in excess of 400 p.s.i. is more expensive and requires more sophisticated equipment; however, it can readily be achieved if a given application requires.

The coolant flow rate must be at least 20 gallons of coolant per minute per square foot of surface over the area of impact, and preferably between 50 and 200 gallons per minute.

The droplets are sprayed at a velocity of from to 300 feet per second. The pressure, distance and velocity of the droplets stated above are essential in order to penetrate the vapor barrier which is formed at the surface of the metal by a vaporization of the aqueous liquid.

In accordance with the present invention it has been found that a surprising super quenching effect is obtained, i.e., the entire surface of the metal is cooled at a surprising rate of at least F. per second, and generally rates in excess of 250 or 300 F. per second. At pressures above 300 psi, cooling rates higher than 400 F. per second are obtained. These rates are determined on an instantaneous basis, i.e., these rates are determined at the instant the surface of the object is at 900 F. In other words, at the very instant the object is at 900 F. the object is cooling at at least this rate. These rates can be calculated by conventional methods or may be obtained from a plot of accurately measured surface temperature as a function of quenching time.

The present invention is particularly applicable to aluminum or copper, strip, sheet or extrusions and may be most advantageously employed where said object is moving. In a commercial operation, it is preferred to utilize a plurality of spaced aqueous jets, particularly where large objects are employed.

The present invention is applicable to any aluminum, copper, aluminum base alloy or copper base alloy article. Furthermore, the cooling medium which is employed is any water base cooling medium. Agents may be added to the water or the water may be admixed with other cooling media in accordance with this invention in order to obtain particular results. For example, where it is desirable to particular cooling operations to avoid chemical reaction with or staining of a surface, chemical inhibitor additives may be included with the water media. Similarly, surfactants or similar type substances may be employed.

Water is naturally the preferred cooling media since this is the conventional cooling media used in commercial operations and it has a particularly high latent heat of evaporation of approximately 1000 British thermal units per pound of water.

The present invention will be more readily apparent from a consideration of the following illustrative examples.

Example I An aluminum plate /2 inch in thickness and 6 inches square was insulated on the back and was fitted with thermocouples as follows These thermocouples were attached so that they could continuously measure and record the temperatures at the front and back of the plate.

The insulated plate including thermocouples was then heated to a temperature of about 1000 F. The plate was sprayed with water in the form of small droplets at a pressure of about 275 p.s.i. and velocity of about 150 feet per second. The Water was sprayed on the surface of the plate in the form of small droplets to penetrate .the vapor barrier found at the surface of the plate by vaporization of the liquid. The initial temperature of the water was about 90 F. The water flowed at a rate of approximately 100 gallons per minute per square foot of surface over the area of impact.

Both the front and back surfaces were cooled to less than 300 F. in less than 3% seconds despite the fact that no water was sprayed on the back and the back was covered with insulation. In contrast, the same sample insulated in the same way, was cooled by immersing in Water maintained at about 90 F. The front surface reached 300 F. in about 5 seconds and the back side did not reach 300 F. until about 7 seconds.

Further, the instantaneous cooling rate at about 900 F. of the sprayed sample was 290 F. per second for the front surface whereas for the immersion quenched sample, the cooling rate at 900 F. was about 50 F. per second.

Example II A steel plate of the same dimensions as the aluminum plate shown in Example I was insulated and fitted with thermocouples in the same manner as in Example I. The procedure in this experiment was the same as in Example 1, except for the use of a steel plate rather than an aluminum plate. The plate was then heated to a temperature of about 1000 F. and cooled with a spray of small droplets as in Example I.

The temperatures on the front and back of the steel plate were continuously measured and recorded. The front surface was cooled to less than 300 F. in about 6 seconds whereas the back took considerably longer and in fact at the end of six seconds the back was still at about 600 F. The instantaneous cooling rate on the front surface was 650 F. per second.

The data should be sharply contrasted with the data in Example I, especially it should be noted that the back of the steel plate is cooled at a very slow rate compared to the front due to the relatively poor thermal diffusivity of the steel. This is true despite the very rapid cooling rate on the front surface.

Example Ill The procedure of Example I was repeated under the same spraying conditions, except that the spray pressure was varied. The cooling rates are shown in the following table:

TABLE I Spray Pressure, Instantaneous Cooling p.s.i. Rate at 900 F., in

seconds The sharp increase in cooling rate for pressures in excess of 150 p.s.i. should be noted and shows the importance of spraying at high pressures. It is surprising that in the lower pressure ranges little increase in cooling rate is obtained; whereas in marked contrast the higher pressures show significant increase in cooling rate.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

What is claimed is:

1. A process for cooling a non-ferrous metal selected from the group consisting of aluminum and copper which comprises:

A. providing said metal wherein the entire surface thereof is at a temperature in excess of 700 F;

B. spraying substantially the entire surface thereof with an aqueous jet of high velocity liquid in the form of droplets to penetrate the vapor barrier formed at the surface of said metal by vaporization of said liquid, wherein (1) said jet is at a distance of from 6 to 24 inches from the surface of the metal,

(2) said jet is sprayed at a pressure of 150 to 600 pounds per square inch,

(2) said droplets have a velocity of from 100 to 300 feet per second, and

(4) the coolant flow rate is at least 20 gallons per minute per square foot of surface over the area of impact;

C. thereby cooling the entire surface of said metal at a rate of at least 150 F. per sec-0nd.

2. A process according to claim 1 wherein said metal is aluminum.

3. A process according to claim 1 wherein said metal is copper.

4. A process according to claim 1 wherein said metal is in strip form.

5. A process according to claim 1 wherein said metal is in extrusion form.

6. A process according to claim 1 wherein said metal is sheet.

7. A process according to claim 1 wherein said metal is initially provided at a temperature of at least 800 F'.

8. A process according to claim 1 wherein said metal is moving.

9. A process according to claim 1 wherein a plurality of spaced, aqueous jets are employed.

10. A process according to claim 1 wherein said pressure is 200 to 400 pounds per square inch.

11. A process according to claim 1 wherein said distance is 6 to 15 inches.

12. A process according to claim 1 wherein said flow rate is between 50 and 200 gallons per minute.

13. A process for cooling a non-ferrous metal selected from the group consisting of aluminum and copper which comprises:

A. providing said metal wherein the entire surface thereof is at a temperature in excess of 800 F.;

B. spraying substantially the entire surface thereotf with an aqueous jet of high velocity liquid in the form of droplets to penetrate the vapor barrier formed at the surface of said metal by vaporization of said liquid, wherein (1) said jet is at a distance of from 6 to 15 inches from the surface of the metal,

(2) said jet is sprayed at a pressure of 200 to 400 pounds per square inch,

(3) said droplets have a velocity of from 100 to 300 feet per second, and

(4) the coolant flow rate is between 50 and 200 gallons per minute per square foot of surface over the area of impact;

C. thereby cooling the entire surface of said metal at a rate of at least 150 F. per second.

References Cited UNITED STATES PATENTS FOREIGN PATENTS Great Britain.

ROBERT A. OLEARY, Primary Examiner.

CHARLES SUKALO, Examiner. 

1. A PROCESS FOR COOLING A NON-FERROUS METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND COPPER WHICH COMPRISES: A. PROVIDING SAID METAL WHEREIN TH ENTIRE SURFACE THEREOF IS AT A TEMPERATURE IN EXCESS OF 700*F; B. SPRAYING SUBSTANTIALLY THE ENTIRE SURFACE THEREOF WITH AN AQUEOUS JET OF HIGH VELOCITY LIQUID IN THE FORM OF DROPLETS TO PENETRATE THE VAPOR BARRIER FORMED AT THE SURFACE OF SAID METAL BY VAPORIZATION OF SAID LIQUID, WHEREIN (1) SAID JET IS AT A DISTANCE OF FROM 6 TO 24 INCHES FROM THE SURFACE OF THE METAL, (2) SAID JET IS SPRAYED AT A PRESSURE OF 150 TO 600 POUNDS PER SQUARE INCH, (2) SAID DROPLETS HAVE A VELOCITY OF FROM 100 TO 300 FEET PER SECOND, AND (4) THE COOLANT FLOW RATE IS AT LEAT 20 GALLONS PER MINUTE PER SQUARE FOOT OF SURFACE OVER THE AREA OF IMPACT C. THEREBY COOLING THE ENTIRE SURFACE OF SAID METAL AT A RATE OF AT LEAST 150*F. PER SECOND. 